In this proposal, the experimental protocols are designed to test the hypothesis that the physiological, biochemical and morphological features of the "blood-brain barrier" are determined; by important and complex interactions between the vascular endothelium and cells that come in contact with it, namely astrocytes, monocyte-macrophages and smooth muscle. These interactions are orderly and are required for the development, maintenance and regeneration after injury of the barrier properties. By combining in vivo studies of the regeneration of the endothelium and return of barrier properties in the model of cerebral freeze-injury with in vitro studies of the isolated endothelial cells alone or in combination with other cells such as astrocytes on either side of a filter in a two chamber system, we should be able to ask specific questions, isolate events, derive quantitative data and correlate the significance of the observation in both the in vivo and in vitro systems.